1. Technical Field
The present invention relates to an optical tomographic observation apparatus and an optical tomographic observation method.
2. Background Art
In recent years, optical coherence tomography (OCT) has been drawing attention that acquires an image reflecting the surface structure or the inner structure of a target to be measured, using a light beam (See Patent Document 1). As the OCT is not invasive on the human body, it is expected to be applied to the medical field and the biological logical field, in particular. In the ophthalmological field, apparatuses for forming images of fundus or corneas have been put into practical use. In the OCT, a light beam from a light source is split into two: a signal beam to irradiate a target to be measured, and a reference beam to be reflected by a reference beam mirror without irradiating the target to be measured, and then a reflected signal beam from the target to be measured is multiplexed with and is caused to interfere with the reference beam, whereby an interference signal is obtained.
The OCT is broadly divided into the time domain OCT and the Fourier domain OCT depending on the method of moving a measurement position in the optical axis direction (hereinafter referred to as z-scan). FIG. 1 shows a schematic diagram of the optics of the time domain OCT. In this scheme, z-scan is performed by using a low coherence light source as a light source and moving a reference beam mirror during the measurement. Accordingly, only the components in a signal beam that have the same optical path lengths as the reference beam will interfere with the reference beam, whereby a detection signal such as the one shown in FIG. 2 is obtained. When envelope detection is performed on the signal shown in FIG. 2, a desired signal such as the one shown in FIG. 3 is obtained through demodulation.
Meanwhile, the Fourier domain OCT is further divided into the wavelength-scanning OCT and the spectral domain OCT. In the wavelength-scanning OCT, z-scan is performed by using a wavelength-scanning light source capable of causing the wavelength of an output beam to scan, and causing the wavelength to scan during the measurement. When Fourier transform is performed on the wavelength dependence of the detected interference beam intensity (i.e., an interference spectrum), a desired signal is obtained through demodulation.
In the spectrum domain OCT, the following corresponds to the z-scan: using a broadband light source for a light source, splitting a generated interference beam using a beam splitter, and detecting the interference beam intensity (i.e., an interference spectrum) for each wavelength component. When Fourier transform is performed on the obtained interference spectrum, a desired signal is obtained through demodulation.
In the conventional OCT apparatuses such as those described above, the spatial resolution in the depth direction is determined by the spectral bandwidth of the light source. Thus, an increase in the resolution has been attempted by widening the spectral bandwidth.